FBAR devices that incorporate one or more film bulk acoustic resonators (FBARs) form part of an ever-widening variety of electronic products, especially wireless products. For example, modern cellular telephones incorporate a duplexer in which each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is an FBAR. A duplexer incorporating FBARs is disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated into this disclosure by reference. Such duplexer is composed of a transmitter band-pass filter connected in series between the output of the transmitter and the antenna and a receiver band-pass filter connected in series with 90° phase-shifter between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another. Ladder filters based on FBARs are also used in other applications.
FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter 10 suitable for use as the transmitter band-pass filter of a duplexer. The transmitter band-pass filter is composed of series FBARs 12 and shunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have a higher resonant frequency than shunt FBARs 14.
FIG. 2 shows an exemplary embodiment 30 of an FBAR. FBAR 30 is composed a pair of electrodes 32 and 34 and a piezoelectric element 36 between the electrodes. The piezoelectric element and electrodes are suspended over a cavity 44 defined in a substrate 42. This way of suspending the FBAR allows the FBAR to resonate mechanically in response to an electrical signal applied between the electrodes.
Above-mentioned U.S. patent application Ser. No. 10/699,289, of which this application is a Continuation-in-Part discloses a band-pass filter that incorporates a decoupled stacked bulk acoustic resonator (DSBAR) composed of a lower FBAR, an upper FBAR stacked on lower FBAR and an acoustic decoupler between the FBARs. Each of the FBARs is composed of a pair of electrodes and a piezoelectric element between the electrodes. An electrical input signal is applied between electrodes of the lower FBAR and the upper FBAR provides a band-pass filtered electrical output signal between its electrodes. The electrical input signal may alternatively be applied between the electrodes of the upper FBAR, in which case, the electrical output signal is taken from the electrodes of the lower FBAR.
Above-mentioned U.S. patent application Ser. No. 10/699,481, of which this disclosure is a Continuation-in-Part, discloses a film acoustically-coupled transformer (FACT) composed of two decoupled stacked bulk acoustic resonators (DSBARs). A first electrical circuit interconnects the lower FBARs of the DSBARs in series or in parallel. A second electrical circuit interconnects the upper FBARs of the DSBARs in series or in parallel. Balanced or unbalanced FACT embodiments having impedance transformation ratios of 1:1 or 1:4 can be obtained, depending on the configurations of the electrical circuits. Such FACTs also provide galvanic isolation between the first electrical circuit and the second electrical circuit.
In the above-described DSBARs and FACTs, each lower FBAR is suspended over a cavity in a substrate similar to the cavity 44 described above with reference to FIG. 2. The cavity allows the constituent FBARs to resonate mechanically in response to an electrical signal applied between the electrodes of one or more of the FBARs.
The FBAR described above with reference to FIG. 2 and devices, such as ladder filters, DSBARs and FACTs, incorporating one or more FBARs will be referred to generically in this disclosure as FBAR devices.
Practical embodiments of FBAR devices are made by forming a cavity in a rigid substrate, such as a silicon substrate, filling the cavity with sacrificial material, planarizing the surface of the substrate, and depositing and patterning the respective layers of the FBAR device on the surface of the sacrificial material, as described in above-mentioned above-mentioned U.S. patent application Ser. No. 10/699,298, for example, but parts of the surface of the sacrificial material remain exposed. Portions of at least the layer of piezoelectric material that provides the piezoelectric element 116 additionally overlap the substrate outside the cavity. After all the layers have been deposited and patterned, a release etch is performed to remove the sacrificial material from the cavity. This leaves the FBAR device suspended over the cavity as shown in FIG. 2.
The need to perform a release etch at the end of the fabrication process limits the choice of materials that can be used to form the substrate, electrodes and piezoelectric element(s) of the FBAR device to materials that are etch compatible with the release etch. It is sometimes desirable not to be subject to this constraint. Moreover, even when etch-compatible materials are used, the release etch can cause separation between the layers of the FBAR device with a consequent impairment of the performance. Accordingly, an alternative way of making FBAR devices that does not involve performing a release etch is desired.
U.S. Pat. No. 6,107,721 of Lakin discloses an FBAR device with an acoustic reflector interposed between the FBAR device and the substrate. No release etch is required in the fabrication of this device. The acoustic reflector is based on a Bragg reflector and is composed of alternating layers silicon dioxide and non-piezoelectric aluminum nitride. In the example disclosed by Lakin, the acoustic reflector had nine layers. Lakin further indicates that more or fewer layers can be used.
The need to deposit nine or more additional layers of material to form the acoustic reflector significantly complicates the process of fabricating the FBAR device notwithstanding the lack of a release etch. Moreover, attempts to simplify the process by reducing the number of layers results in an FBAR device whose frequency response exhibits undesirable spurious artifacts. The artifacts result from to the reduced isolation provided by the reduced number of layers allowing the FBAR device to interact mechanically with the substrate.
Some commercially-available FBAR devices incorporate an acoustic reflector composed of alternating layers of silicon dioxide and a metal. However, the frequency responses of such FBAR devices exhibit undesirable spurious artifacts, such as additional transmission peaks in the stop band.
What is needed therefore is a way of isolating an FBAR device from the substrate that does not require a release etch, that does not unduly complicate the fabrication process and that provides sufficient isolation between the FBAR device and the substrate that the frequency response of the FBAR device is free from undesirable spurious artifacts.